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A Practical Guide for Health Researchers - part 5

Một thử nghiệm ý nghĩa thống kê ước tính khả năng một kết quả nghiên cứu quan sát, ví dụ như một sự khác biệt giữa hai nhóm hoặc một hiệp hội, có thể là do để cơ hội và do đó không có suy luận có thể được làm từ nó. | 96 A practical guide for health researchers 8.8.2 Statistical significance A statistical significance test estimates the likelihood that an observed study result for example a difference between two groups or an association can be due to chance and therefore no inference can be made from it. Tests of statistical significance are based on common logic and common sense. That a difference is likely to be real and not due to chance is based largely on three criteria. The first is the magnitude of the difference observed. It is reasonable to expect that the larger the difference the more likely that it is not due to chance. The second is the degree of variations in the values obtained in the study. If the values fall within too wide a range differences in means would be more likely to be due to chance variations. The third very important criterion is the size of sample studied. The larger the size of the sample the more likely that the result drawn from it will reflect the results in the population. What statisticians do is to turn this simple logic through mathematics into a quantitative formula to describe the level of probability. When the data are analysed we set an arbitrary value for what we can accept as alpha or level of statistical significance i.e. the probability of committing a type I error rejecting the null hypothesis when it is actually true or proving an association when none exists . The statistical tests then determine the P value. P is the probability that a difference or an association as large as the one observed could have occurred by chance alone. The null hypothesis is rejected if the P value is less than alpha the predetermined level of statistical significance. Probability or P is usually expressed as a percentage. A result is commonly considered to be unlikely to be due to chance or to be statistically significant if the P value is less than 5 P less than 0.05 and is said to be highly significant if P is less than 0.01. There is nothing .